Fix ignored doc tests

There's a bunch of doc tests which are marked to be ignored, for example:

https://github.com/antonio-hickey/tradestation-rs/blob/c3a9d730398ea83b632177e4ab32b37ce731ad02/src/execution.rs#L74-L107

These need some slight refactoring so that they compile and pass correctly. We still want some partial ignoring like runtime tests for most of them, because they have fake placeholder credentials in the tests. They can't be tested without mocking the HTTP request's which is out of scope for doc examples.

See this commit for an example: https://github.com/antonio-hickey/tradestation-rs/commit/fd79d4c25d70eef9ff5e6b6db66db134ac4fee94