Allow overriding HTTP client of a Spine Web JavaScript client.

[YegorUdovchenko]

The current version of a Spine Web client for JavaScript provides a high-level API for interacting with a Spine-based backend and carries all HTTP-related functionality. It makes tricky to:

• add custom HTTP headers, e.g. "Authorization";
• use non-CORS request mode;
• expand client to interact with custom backend endpoints (non "/command" or "/query" ) that use similar protocol, e.i. Protobuf request/response body and the same set of auth headers;

To solve above-mentioned issues in Pure Photos we use our custom implementation of HttpClient and inject it after Spine Web client instantiation.

[armiol]

@YegorUdovchenko I am not sure whether this is an old issue, or a new one. But just in case: #194.

[armiol]

It's been released with 1.9.0.

[YegorUdovchenko]

@YegorUdovchenko I am not sure whether this is an old issue, or a new one. But just in case: #194.

Sorry, missed this update.