Fix multipart to set its header even when other headers are provided

`@raw_request.initialize_http_header` resets the headers, so when
options contained :headers, it always erased the Content-Type header set
for multipart.

Now the Content-Type header is always set to "multipart/form-data" when
body is detected as multipart. This is IMHO not correct, but that's how
the original implementation was designed...

lib/httparty/request.rb

@@ -205,15 +205,6 @@ module HTTParty
       @raw_request = http_method.new(request_uri(uri))
       @raw_request.body_stream = options[:body_stream] if options[:body_stream]
 
-      if options[:body]
-        body = Body.new(options[:body], query_string_normalizer: query_string_normalizer)
-        if body.multipart?
-          content_type = "multipart/form-data; boundary=#{body.boundary}"
-          @raw_request.initialize_http_header('Content-Type' => content_type)
-        end
-        @raw_request.body = body.call
-      end
-
       if options[:headers].respond_to?(:to_hash)
         headers_hash = options[:headers].to_hash
 
@@ -227,6 +218,15 @@ module HTTParty
         end
       end
 
+      if options[:body]
+        body = Body.new(options[:body], query_string_normalizer: query_string_normalizer)
+        if body.multipart?
+          content_type = "multipart/form-data; boundary=#{body.boundary}"
+          @raw_request['Content-Type'] = content_type
+        end
+        @raw_request.body = body.call
+      end
+
       if options[:basic_auth] && send_authorization_header?
         @raw_request.basic_auth(username, password)
         @credentials_sent = true

spec/httparty/request_spec.rb

@@ -342,6 +342,27 @@ RSpec.describe HTTParty::Request do
         expect(CGI.unescape(body)).to eq("foo=bar&foo=baz&page=1")
       end
     end
+
+    context "when body is multipart" do
+      it "sets header Content-Type: multipart/form-data; boundary=" do
+        @request.options[:body] = {file: File.open(File::NULL, 'r')}
+        @request.send(:setup_raw_request)
+        headers = @request.instance_variable_get(:@raw_request).each_header.to_a
+        headers = Hash[*headers.flatten]  # Ruby 2.0 doesn't have Array#to_h
+        expect(headers['content-type']).to match(%r{^multipart/form-data; boundary=---})
+      end
+
+      context "and header Content-Type is provided" do
+        it "overwrites the header to: multipart/form-data; boundary=" do
+          @request.options[:body] = {file: File.open(File::NULL, 'r')}
+          @request.options[:headers] = {'Content-Type' => 'application/x-www-form-urlencoded'}
+          @request.send(:setup_raw_request)
+          headers = @request.instance_variable_get(:@raw_request).each_header.to_a
+          headers = Hash[*headers.flatten]  # Ruby 2.0 doesn't have Array#to_h
+          expect(headers['content-type']).to match(%r{^multipart/form-data; boundary=---})
+        end
+      end
+    end
   end
 
   describe 'http' do