Use strip & empty? to detect Whitespaces

## Test code
```ruby
require 'benchmark/ips'

WhitespaceOrNothing = /\A\s*\z/
token = " " * 20

Benchmark.ips do |x|
  x.report("WhitespaceOrNothing") {
    token =~ WhitespaceOrNothing
  }
  x.report("strip & empty?") {
    token.strip.empty?
  }

  x.compare!
end
```

## Result
```
Warming up --------------------------------------
 WhitespaceOrNothing   266.391k i/100ms
      strip & empty?     1.044M i/100ms
Calculating -------------------------------------
 WhitespaceOrNothing      2.705M (± 0.4%) i/s -     13.586M in   5.023453s
      strip & empty?     10.400M (± 1.1%) i/s -     52.182M in   5.017990s

Comparison:
      strip & empty?: 10400286.2 i/s
 WhitespaceOrNothing:  2704552.3 i/s - 3.85x  (± 0.00) slower
```

lib/liquid/block_body.rb

@@ -7,7 +7,6 @@ module Liquid
     LiquidTagToken      = /\A\s*(\w+)\s*(.*?)\z/o
     FullToken           = /\A#{TagStart}#{WhitespaceControl}?(\s*)(\w+)(\s*)(.*?)#{WhitespaceControl}?#{TagEnd}\z/om
     ContentOfVariable   = /\A#{VariableStart}#{WhitespaceControl}?(.*?)#{WhitespaceControl}?#{VariableEnd}\z/om
-    WhitespaceOrNothing = /\A\s*\z/
     TAGSTART            = "{%"
     VARSTART            = "{{"
 
@@ -37,7 +36,7 @@ module Liquid
 
     private def parse_for_liquid_tag(tokenizer, parse_context)
       while (token = tokenizer.shift)
-        unless token.empty? || token =~ WhitespaceOrNothing
+        unless token.strip.empty?
           unless token =~ LiquidTagToken
             # line isn't empty but didn't match tag syntax, yield and let the
             # caller raise a syntax error
@@ -150,7 +149,7 @@ module Liquid
           end
           parse_context.trim_whitespace = false
           @nodelist << token
-          @blank &&= !!(token =~ WhitespaceOrNothing)
+          @blank &&= token.strip.empty?
         end
         parse_context.line_number = tokenizer.line_number
       end