Make StringFieldMapper.toXContent aware of defaults for not_analyzed fields.

StringFieldMapper.toXContent uses the defaults for analyzed fields in order to
know which options to add to the builder. This means that if the field is not
analyzed and has norms enabled, it will omit to emit `norms.enabled: true`.
Parsing the mapping again will result in a StringFieldMapper that has norms
disabled.

The same fix applies to index options.

Close #4760