Retain source location from log crate if they are &'static

[SpriteOvO]

Fixes #22.

The problem is that the source location from the log crate is an internal type MaybeStaticStr<'a> which is like a Cow<'static, str>, so that means they are possible to be runtime &'a str instead of &'static str.

I had left a None for the source_location inside the proxy since we store location information as &'static str so there is no way to store runtime &'a str in it unless we change the API.

But reconsider the situation, most crates only use the logging macros from log crate instead of constructing something manually, and source location from macros should always be &'static str, So we can safely ignore those &'a str cases, which should not be a big problem.