As discussed in rouge-ruby#1097, the Go lexer tokenises whitespace using the
`Other` token. This causes issues when rendering in HTML the code parsed
by Rouge because:

1. the use of `Other` will result in the whitespace being wrapped in
   separate HTML tags, which in turn causes
2. all newlines to be collapsed (due to CSS rules that expect newlines
   not to be wrapped in seperate HTML tags).

The `Other` token's intended use is described as follows:

> Token for data not matched by a parser (e.g. HTML markup in PHP code)

This is not a correct characterisation of whitespace in Go and is not
consistent with other lexers. This commit changes the token to `Text`,
the token generally used by other lexers. It fixes rouge-ruby#1097.