fix: requests can be indented ( by spaces or tabs)

[UndeadKernel]

The regexp in restclient-method-url-regexp can be modified to support REST requests that start with spaces or tabs. Before this commit, if a request starts with a space, it will fail.

@pashky, do you think this "feature" makes sense? This allows me to indent my requests in, for example, org buffers.

[UndeadKernel]

Hey @pashky. Sorry to bother you, but would you take a look at this PR if you believe it is valid? If you don't think it's worth adding it, I'd like to close the PR and instead modify a local copy of restclient.el using el-patch.

Cheers!