added content-type filtering to decoders

lib/httpx/plugins/multipart.rb

@@ -58,9 +58,16 @@ module HTTPX
         end
 
         def decode(response)
-          return Transcoder::Form.decode(response) if response.headers["content-type"] == "application/x-www-form-urlencoded"
+          content_type = response.content_type.mime_type
 
-          Decoder.new(response)
+          case content_type
+          when "application/x-www-form-urlencoded"
+            Transcoder::Form.decode(response)
+          when "multipart/form-data"
+            Decoder.new(response)
+          else
+            raise Error, "invalid form mime type (#{content_type})"
+          end
         end
 
         def multipart?(data)

lib/httpx/transcoder/form.rb

@@ -47,7 +47,11 @@ module HTTPX::Transcoder
       Encoder.new(form)
     end
 
-    def decode(_response)
+    def decode(response)
+      content_type = response.content_type.mime_type
+
+      raise Error, "invalid form mime type (#{content_type})" unless content_type == "application/x-www-form-urlencoded"
+
       Decoder
     end
   end

lib/httpx/transcoder/json.rb

@@ -5,6 +5,10 @@ require "json"
 
 module HTTPX::Transcoder
   module JSON
+    JSON_REGEX = %r{\bapplication/(?:vnd\.api\+)?json\b}i.freeze
+
+    using HTTPX::RegexpExtensions unless Regexp.method_defined?(:match?)
+
     module_function
 
     class Encoder
@@ -28,7 +32,11 @@ module HTTPX::Transcoder
       Encoder.new(json)
     end
 
-    def decode(_response)
+    def decode(response)
+      content_type = response.content_type.mime_type
+
+      raise Error, "invalid json mime type (#{content_type})" unless JSON_REGEX.match?(content_type)
+
       ::JSON.method(:parse)
     end
   end

test/support/requests/plugins/multipart.rb

@@ -228,7 +228,7 @@ module Requests
                                HTTPX::Request.new(:get, "http://example.com"),
                                200,
                                "2.0",
-                               { "content-type" => "Content-Type: multipart/form-data; boundary=90" }
+                               { "content-type" => "multipart/form-data; boundary=90" }
                              )
         form_response << [
           "--90\r\n",