Add ContentLength property in mocked http.Response

[browny]

When using httpmock with grequests http client, the grequests.Response.String() method will fail to read http response body because the property ContentLength of the mocked http.Response is 0. (When ContentLength == 0, grequests doesn't copy http response body to its own Response, refer to here)

According to the godoc of http.Response, I think make it as -1 would be good.

// ContentLength records the length of the associated content. The // value -1 indicates that the length is unknown. Unless Request.Method // is "HEAD", values >= 0 indicate that the given number of bytes may // be read from Body.

thanks :)

[dlebech]

@browny Thanks for the PR! I have two requests:

1. Given that you have access to the actual response body when setting this value, I think it would be better to set the correct ContentLength based on the length of the string and the byte slice.
2. If you could also add a test that asserts the correct length and possibly test this with either something like io.CopyN() or simply resp.Body.Len() == resp.ContentLength. Something along those lines would be awesome :smiley: