hardcode authority header for mixer temporarily

[lizan]

What this PR does / why we need it:
Fixes #1737

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None

[istio-testing]

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[kyessenov]

Thanks. @qiwzhang the callback is called after prepareHeaders is called, which is the one guilty of setting the header.

[mandarjog]

Thanks Lizan!

[mandarjog]

Lizan

W0509 20:56:30.945]     ~~~~~~~~~~~~~~~~~^~~~~
W0509 20:56:30.945] bazel-out/k8-fastbuild/bin/external/envoy/include/envoy/http/_virtual_includes/header_map_interface/envoy/http/header_map.h:223:8: note: declared private here
W0509 20:56:30.945]   void value(const char*); // Do not allow auto conversion to std::string