Stacktrace: don't clobber filename if it was explicitly provided

I'm using the abs_path to provide a URL to github... so it makes things look ugly when Sentry clobbers the filename I send it. e.g.:

            {
              "function": "main",
              "abs_path": "https://github.com/org/repo/blob/da39a3ee5e6b4b0d3255bfef95601890afd80709/path/to/main.m#L16",
              "in_app": true,
              "lineno": 16,
              "filename": "/org/repo/blob/da39a3ee5e6b4b0d3255bfef95601890afd80709/path/to/main.m"
            },

So yeah, I'd rather Sentry retain the filename that was sent in with the event (which was just "main.m").