Update documented default values for http1::Builder

[Will-Low]

1. Document the default value for allow_multiple_spaces_in_request_line_delimiters.
2. Update the formatting of the default boolean values to be monospaced. This is used by half_close and I personally found it easier to read, since the default values pop out. If it's preferred to standardize to non-monospace, that's fine by me too, as long as it's consistent between all methods in the page.

[seanmonstar]

Nice improvement, thanks!