add support for the google.api.HttpBody proto as a request

[jinleileiking]

My request is:

curl http://xxx.xxx -d "id=aaaa&starttime=111"

[jinleileiking]

https://github.com/googleapis/googleapis/blob/master/google/api/httpbody.proto

[jinleileiking]

136b143

[jinleileiking]

#458

[theRealWardo]

I think your request is actually a dupe of #7

[jinleileiking]

@theRealWardo Your code does not show where does the code put in. jinleileiking@bb153b3
I put to the right place

[jinleileiking]

@yugui @tmc Is it the good way to finish this feature?

My solustion seems not good. Any advice?