Strip newlines from base-64-encoded auth string.

`Base64.encode64` [adds newlines every 60 chars](http://www.ruby-doc.org/stdlib-1.9.3/libdoc/base64/rdoc/Base64.html#method-i-encode64), so when used with long enough credentials strings, this currently fails to generate a correct `Authorization` header.

This change just regexes the newlines out. A neater solution might be to use [`strict_encode64`](http://www.ruby-doc.org/stdlib-1.9.3/libdoc/base64/rdoc/Base64.html#method-i-strict_encode64) but its documented strictness around padding & alphabet might be a problem for some, I guess.