Reduce allocations during mail gem's specs

gistfile1.txt

Before:
Top 10 allocation sites:
  String allocations at <PWD>/lib/mail/utilities.rb:180
    4639 allocations during ./spec/mail/attachments_list_spec.rb:312
    ...
  Array allocations at <PWD>/lib/mail/parsers/received_parser.rb:13
    2326 allocations during ./spec/mail/message_spec.rb:1627
    ...
  String allocations at <PWD>/lib/mail/fields/common/common_field.rb:46
    2269 allocations during ./spec/mail/fields/content_type_field_spec.rb:198
    ...
  Array allocations at <PWD>/lib/mail/parsers/content_type_parser.rb:17
    2268 allocations during ./spec/mail/fields/content_type_field_spec.rb:198
    ...
  String allocations at <PWD>/lib/mail/message.rb:1972
    2188 allocations during ./spec/mail/message_spec.rb:77
    ...
  String allocations at <PWD>/lib/mail/fields/common/parameter_hash.rb:20
    2030 allocations during ./spec/mail/attachments_list_spec.rb:312
    ...
  String allocations at <PWD>/lib/mail/mail.rb:238
    2016 allocations during ./spec/mail/fields/content_type_field_spec.rb:198
    ...
  String allocations at <PWD>/lib/mail/encodings.rb:50
    1756 allocations during ./spec/mail/network/retriever_methods/test_retriever_spec.rb:78
    ...
  String allocations at <PWD>/lib/mail/field.rb:118
    1325 allocations during ./spec/mail/message_spec.rb:77
    ...
  MatchData allocations at <PWD>/lib/mail/utilities.rb:180
    1292 allocations during ./spec/mail/attachments_list_spec.rb:312
    ...
Total allocations: 855530


After changes to 11x files in lib/mail:
Top 10 allocation sites:
  String allocations at <PWD>/lib/mail/field.rb:118
    2168 allocations during ./spec/mail/message_spec.rb:77
    ...
  String allocations at <PWD>/lib/mail/fields/common/parameter_hash.rb:20
    2030 allocations during ./spec/mail/attachments_list_spec.rb:312
    ...
  String allocations at <PWD>/lib/mail/fields/common/common_field.rb:47
    2017 allocations during ./spec/mail/fields/content_type_field_spec.rb:198
    ...
  String allocations at <PWD>/lib/mail/mail.rb:238
    1512 allocations during ./spec/mail/fields/content_type_field_spec.rb:198
    ...
  String allocations at <PWD>/lib/mail/utilities.rb:183
    1331 allocations during ./spec/mail/attachments_list_spec.rb:312
    ...
  String allocations at <PWD>/lib/mail/header.rb:251
    1173 allocations during ./spec/mail/message_spec.rb:77
    ...
  String allocations at <PWD>/lib/mail/field.rb:132
    1121 allocations during ./spec/mail/message_spec.rb:77
    ...
  String allocations at <PWD>/lib/mail/header.rb:259
    1085 allocations during ./spec/mail/message_spec.rb:77
    ...
  Mail::Field allocations at <PWD>/lib/mail/header.rb:92
    1085 allocations during ./spec/mail/message_spec.rb:77
    ...
  MatchData allocations at <PWD>/lib/mail/field.rb:118
    1084 allocations during ./spec/mail/message_spec.rb:77
    ...
Total allocations: 613593