Faster String#blank? (140% to 470% improvement)

Benchmarking code:
```ruby
require 'benchmark/ips'
require 'active_support/all'

class String
  def improved_blank?
    return empty? || strip.empty? if ascii_only?
    BLANK_RE === self
  end
end

puts "EMPTY STRING"
Benchmark.ips do |x|
  x.report('original') { ''.blank? }
  x.report('improved') { ''.improved_blank? }
  x.compare!
end

puts "ASCII STRING"
Benchmark.ips do |x|
  x.report('original') { ' abcd '.blank? }
  x.report('improved') { ' abcd '.improved_blank? }
  x.compare!
end

puts "UNICODE STRING"
Benchmark.ips do |x|
  x.report('original') { "\u00a0".blank? }
  x.report('improved') { "\u00a0".improved_blank? }
  x.compare!
end
```

Results:
```
EMPTY STRING
Calculating -------------------------------------
            original      9173 i/100ms
            improved     15256 i/100ms
-------------------------------------------------
            original   352351.5 (±18.4%) i/s -    1706178 in   5.011738s
            improved  1658752.5 (±21.4%) i/s -    7795816 in   4.998368s

Comparison:
            improved:  1658752.5 i/s
            original:   352351.5 i/s - 4.71x slower

ASCII STRING
Calculating -------------------------------------
            original     11325 i/100ms
            improved     13602 i/100ms
-------------------------------------------------
            original   671560.4 (±16.3%) i/s -    3261600 in   5.001736s
            improved   948691.9 (±20.0%) i/s -    4543068 in   4.994523s

Comparison:
            improved:   948691.9 i/s
            original:   671560.4 i/s - 1.41x slower

UNICODE STRING
Calculating -------------------------------------
            original      9198 i/100ms
            improved      9023 i/100ms
-------------------------------------------------
            original   331457.0 (±18.0%) i/s -    1609650 in   5.013549s
            improved   326054.9 (±17.3%) i/s -    1588048 in   5.020778s

Comparison:
            original:   331457.0 i/s
            improved:   326054.9 i/s - 1.02x slower
```