Allow HttpRequest entity content-type without parameters.

According to the HTTP specification section [3.7](https://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7), older HTTP clients/server may not be able to handle Media Type definition with parameters.
While implementing the test suite for [AWS Signature 4](https://github.com/akka/alpakka/pull/120), I did not found a way to build a HttpRequest, with body's content-type (media type) without parameters.
It is this possible with current implementation?

So, if I build a HttpRequest as:
`
HttpRequest(HttpMethods.getForKey(rawMethod.trim).getOrElse(HttpMethods.CONNECT), encodeIfRequired(rawUri),
headers, HttpEntity(body), HttpProtocols.getForKey(rawProtocol).getOrElse(HttpProtocols.HTTP/1.1))
`

And `headers` variable already contains a Content-Type header, then I'm expecting that this header will not be overwritten by HttpRequest implementation when the HttpEntity is created.

Start by reading the HttpRequest construction path and HttpEntity content-type handling described in the issue. Trace how an existing Content-Type header is treated when the entity is created, then verify that a request can retain a media type without parameters and that the current behavior remains covered by regression testing.

Written by the indexing model from the issue text.