Resolve a bug related to the switch to Sets

Fully resolves #117, #127, and #134.

4 files changed, 21 insertions, 7 deletions

diff --git a/History.md b/History.md
index d910fec..f85ad87 100644
--- a/History.md
+++ b/History.md
@@ -11,10 +11,18 @@
         true` to files so that modern Rubies can automatically reduce duplicate
         string allocations. [#135][]
 
-*   1 bug fix
+*   2 bug fixes
 
     *   Burke Libbey fixed a problem with cached data loading. [#126][]
 
+    *   Resolved an issue where Enumerable#inject returns +nil+ when provided
+        an empty enumerable and a default value has not been provided. This is
+        because when Enumerable#inject isn't provided a starting value, the
+        first value is used as the default value. In every case where this
+        error was happening, the result was supposed to be an array containing
+        Set objects so they can be reduced to a single Set. [#117][], [#127][],
+        [#134][].
+
 *   Deprecations:
 
     *   Lazy loading (`$RUBY_MIME_TYPES_LAZY_LOAD`) has been deprecated.
diff --git a/lib/mime/types.rb b/lib/mime/types.rb
index e3b9f61..fd56908 100644
--- a/lib/mime/types.rb
+++ b/lib/mime/types.rb
@@ -151,7 +151,7 @@ class MIME::Types
   def type_for(filename)
     Array(filename).flat_map { |fn|
       @extension_index[fn.chomp.downcase[/\.?([^.]*?)$/, 1]]
-    }.compact.inject(:+).sort { |a, b|
+    }.compact.inject(Set.new, :+).sort { |a, b|
       a.priority_compare(b)
     }
   end
@@ -171,7 +171,7 @@ class MIME::Types
         nil
       when MIME::Types
         variants = mime_type.instance_variable_get(:@type_variants)
-        add(*variants.values.inject(:+).to_a, quiet)
+        add(*variants.values.inject(Set.new, :+).to_a, quiet)
       when Array
         add(*mime_type, quiet)
       else
@@ -218,7 +218,7 @@ Type #{type} is already registered as a variant of #{type.simplified}.
   def match(pattern)
     @type_variants.select { |k, _|
       k =~ pattern
-    }.values.inject(:+)
+    }.values.inject(Set.new, :+)
   end
 end
 
diff --git a/lib/mime/types/container.rb b/lib/mime/types/container.rb
index 176ae3e..abccc2e 100644
--- a/lib/mime/types/container.rb
+++ b/lib/mime/types/container.rb
@@ -3,9 +3,9 @@
 require 'set'
 
 # MIME::Types requires a container Hash with a default values for keys
-# resulting in an empty array (<tt>[]</tt>), but this cannot be dumped through
-# Marshal because of the presence of that default Proc. This class exists
-# solely to satisfy that need.
+# resulting in an empty Set, but this cannot be dumped through Marshal because
+# of the presence of that default Proc. This class exists solely to satisfy
+# that need.
 class MIME::Types::Container < Hash # :nodoc:
   def initialize
     super
diff --git a/test/test_mime_types.rb b/test/test_mime_types.rb
index 4ebb29d..111f771 100644
--- a/test/test_mime_types.rb
+++ b/test/test_mime_types.rb
@@ -85,6 +85,12 @@ describe MIME::Types do
       refute_empty mime_types[/gzip/, registered: true]
       refute_equal mime_types[/gzip/], mime_types[/gzip/, registered: true]
     end
+
+    it 'properly returns an empty result on a regular expression miss' do
+      assert_empty mime_types[/^foo/]
+      assert_empty mime_types[/^foo/, registered: true]
+      assert_empty mime_types[/^foo/, complete: true]
+    end
   end
 
   describe '#add' do