Added support for custom headers

[arjuninv]

Added support for adding custom headers after creating a ClientStub object.

Reason
To be able to add headers such as auth tokens from third party authentication services that may not necessarily be available at the time of creation of the ClientStub and Client object.

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This change is 

[paulftw]

this would erase options.headers if it exists.
Something that clients may rely on, e.g. Ratel

if (this.options.headers === undefined) {
 this.options.headers = {};
}

would be much safer and backwards compatible

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