Fix. the issue #346 on igrigorik/em-http-request

[jabbadev]

i had to apply this modify to fix the problem of the #346 issue.

with jruby 9.2 and http_parser.rb v0.6.0, thats instructions fails.

@p = Http::Parser.new
@p.header_value_type = :mixed

I think that the parser has a default header_value_type as "mixed" and is unnecessary the explicit assigment