Unacceptable value for Protobuf::Field::String field

Protocol file:

```
package search;

message VideoQuery {
  required string query = 1;
}
```

Generated file:

```
##
# This file is auto-generated. DO NOT EDIT!
#
require 'protobuf/message'

module Search
  
  ##
  # Message Classes
  #
  class VideoQuery < ::Protobuf::Message; end
  
  ##
  # Message Fields
  #
  class VideoQuery
    required ::Protobuf::Field::StringField, :query, 1
  end
  
end
```

Code throwing error:
```
[4] pry(main)> query = Search::VideoQuery.new                                                                                                               
=> {}
[5] pry(main)> query.query = "A"                                                                                                                            
TypeError: unacceptable value A for type Protobuf::Field::StringField field query
from /Users/jeremy/.rvm/gems/ruby-1.9.3-p286/gems/protobuf-2.6.2/lib/protobuf/field/bytes_field.rb:40:in `rescue in block (2 levels) in define_setter'
```