Do not add a `Content-Type` header to the HTTP upgrade request

This patch was authored by @fumoboy007 and released by @0xTim.

⚠️ WARNING: This release removes the hard-coded content-type header from the upgrade request. If you were relying on this you'll need to manually add it now, but since there's no body it shouldn't be required

Some WebSocket servers use the Content-Type header to determine which serialization format to use for its WebSocket messages. However, HTTPInitialRequestHandler adds a hard-coded Content-Type: text/plain; charset=utf-8 header to the WebSocket client’s HTTP upgrade request, so the client cannot customize the header value.

Given that the HTTP upgrade request body is empty, there is no need for HTTPInitialRequestHandler to add the Content-Type header. This will allow the client to add their own, if desired.

Fixes #126.