Add setter for Context

In d9259e3 a getter was added
for the context, this adds a setter.

cc @LucasRoesler

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Author: alexellis

handler.go

@@ -33,6 +33,17 @@ func (r *Request) Context() context.Context {
 	return r.ctx
 }
 
+// WithContext overides the context for the Request struct
+func (r *Request) WithContext(ctx context.Context) {
+	// AE: Not keen on panic mid-flow in user-code, however stdlib also appears to do
+	// this. https://golang.org/src/net/http/request.go
+	// This is not setting a precedent for broader use of "panic" to handle errors.
+	if ctx == nil {
+		panic("nil context")
+	}
+	r.ctx = ctx
+}
+
 // FunctionHandler used for a serverless Go method invocation
 type FunctionHandler interface {
 	Handle(req Request) (Response, error)