Save a string allocation inside loop

In the `tag_options` method, strings are continuously added to the
`output` string. Previously, we concatenated two strings and added the
generated string to `output`. By adding each of the strings to
`output`, one after the other, we will save the allocation of that
concatenated string.

Benchmark:

    require 'benchmark/ips'

    sep = " ".freeze

    Benchmark.ips do |x|
      x.report("string +") {
        output = ""
        output << sep + "foo"
      }
      x.report("string <<") {
        output = ""
        output << sep
        output << "foo"
      }
      x.compare!
    end

Results (Ruby 2.2.2):

    Calculating -------------------------------------
                string +    88.086k i/100ms
               string <<    94.287k i/100ms
    -------------------------------------------------
                string +      2.407M (± 5.8%) i/s -     12.068M
               string <<      2.591M (± 7.0%) i/s -     12.917M

    Comparison:
               string <<:  2591482.4 i/s
                string +:  2406883.7 i/s - 1.08x slower

actionview/lib/action_view/helpers/tag_helper.rb

@@ -154,12 +154,17 @@ def tag_options(options, escape = true)
           options.each_pair do |key, value|
             if TAG_PREFIXES.include?(key) && value.is_a?(Hash)
               value.each_pair do |k, v|
-                output << sep + prefix_tag_option(key, k, v, escape)
+                output << sep
+                output << prefix_tag_option(key, k, v, escape)
               end
             elsif BOOLEAN_ATTRIBUTES.include?(key)
-              output << sep + boolean_tag_option(key) if value
+              if value
+                output << sep
+                output << boolean_tag_option(key)
+              end
             elsif !value.nil?
-              output << sep + tag_option(key, value, escape)
+              output << sep
+              output << tag_option(key, value, escape)
             end
           end
           output unless output.empty?