Add option to ignore content-length with multipart body

[outamaa]

The current implementation of multipart::form requires that the Content-Length header exists. However, there are valid situations where this is not the case, for example when Transfer-Encoding header is present. See e.g. RFC2616, section 4.4.

multipart::form should have an option for not checking maximum content length and therefore not requiring Content-Length headers to be present in the request.

I already made a pull request implementing such an option without breaking the current API.