ODataHttpRequest : headers inherited from the connection arg are not handled

[jblaval]

When executing a request with pyodata's Client, one may need to use some headers, for example for authentication purpose. ODataHttpRequest takes both a Connection argument and a headers keyword-argument, whereas Connection object might already have a headers attribute.
Hence, ODataHttpRequest's init() should merge headers from Connection and from args, overriding the first with the second if necessary.