Handle HEAD requests

[sgohlke]

At the moment GraphQLServer cannot handle requests with HEAD method. In order to do so

• Adjust the allowed methods in GraphQLServer.getRequestInformation
• Add optional "method" field to GraphQLRequestInfo
• Check if the RequestInformationExtractor can read information from the request, add method to returned GraphQLRequestInfo
• Add optional "method" field to ResponseParameters
• Add method param to calls of sendResponse
• In sendResponse check if method is HEAD. If so, get the length of the body and add it to the content-length header, then call response.end without any body.