Add trailing clrf when building header for non-file parts.

When building a `StreamingMultipartBody`, the following example shows the
header for the `first_name` part should be followed by a clrf before the
part's content starts.

==== Before ====
--test-boundary-123
Content-Disposition: form-data; name="avatar"; filename="tiny.gif"
Content-Type: image/gif

GIF89a,;
--test-boundary-123
Content-Disposition: form-data; name="first_name"
John
--test-boundary-123

==== After ====
--test-boundary-123
Content-Disposition: form-data; name="avatar"; filename="tiny.gif"
Content-Type: image/gif

GIF89a,;
--test-boundary-123
Content-Disposition: form-data; name="first_name"

John
--test-boundary-123

Author: serixscorpio

lib/httparty/request/streaming_multipart_body.rb

@@ -135,6 +135,8 @@ def build_part_header(key, value, is_file)
           header << %(; filename="#{file_name(value).gsub(/["\r\n]/, replacement_table)}").b
           header << NEWLINE.b
           header << "Content-Type: #{content_type(value)}#{NEWLINE}".b
+        else
+          header << NEWLINE.b
         end
         header << NEWLINE.b
         header

spec/httparty/request_spec.rb

@@ -282,14 +282,14 @@
         allow(HTTParty::Request::MultipartBoundary).to receive(:generate).and_return('test-boundary-123')
 
         # Get streaming content
-        request1 = HTTParty::Request.new(Net::HTTP::Post, 'http://api.foo.com/v1', body: { avatar: file })
+        request1 = HTTParty::Request.new(Net::HTTP::Post, 'http://api.foo.com/v1', body: { avatar: file, first_name: "John" })
         request1.send(:setup_raw_request)
         streaming_content = request1.instance_variable_get(:@raw_request).body_stream.read
 
         file.rewind
 
         # Get non-streaming content
-        request2 = HTTParty::Request.new(Net::HTTP::Post, 'http://api.foo.com/v1', body: { avatar: file }, stream_body: false)
+        request2 = HTTParty::Request.new(Net::HTTP::Post, 'http://api.foo.com/v1', body: { avatar: file, first_name: "John" }, stream_body: false)
         request2.send(:setup_raw_request)
         non_streaming_content = request2.instance_variable_get(:@raw_request).body