Strip leading/trailing whitespace from CSS exprs

When tokenising CSS expressions we now strip leading and trailing
whitespace from the input string. This is performed without any checks
as a check + `String#strip` ended up being slower compared to just
running `String#strip`. On top of that we cache expressions anyway, so
the overhead of `String#strip` is very small.

Fixes https://gitlab.com/yorickpeterse/oga/issues/187

lib/oga/css/lexer.rl

@@ -23,7 +23,7 @@ module Oga
 
       # @param [String] data The data to lex.
       def initialize(data)
-        @data = data
+        @data = data.strip
       end
 
       # Gathers all the tokens for the input and returns them as an Array.

spec/oga/css/lexer_spec.rb

@@ -0,0 +1,7 @@
+require 'spec_helper'
+
+describe Oga::CSS::Lexer do
+  it 'ignores leading and trailing whitespace' do
+    expect(lex_css(' foo ')).to eq([[:T_IDENT, 'foo']])
+  end
+end