Add String#downcase_first method

[markaschneider]

Rails 5 added String#upcase_first, which converts the first letter of a
string to uppercase (returning a duplicate). This commit adds the
corollary method, converting the first letter to lowercase.

Summary

This adds String#downcasecase_first method via ActiveSupport::Inflector, which
is how String#upcase_first was added.

[markaschneider]

I didn't see any mention of #upcase_first in the Inflections section of the Active Support Core Extentions page. I'm happy to add documentation of both #upcase_first and #downcase_first if desired.

[fatkodima]

Can you share a real-world use case for this method?

[markaschneider]

We wanted to prepend verbiage to a standard message in some cases but not others. Our standard message includes several interpolations, and we wanted to keep things DRY while not introducing a variable in haml and (for better or worse) not creating a helper method.

For example:

en.yml

en:
  exceptions:
    common: Please contact %{support_rep_name} at %{support_email} or %{support_number} if you need immediate assistance.
    403:
      message_prefix: "If you feel you have reached this page in error, "

500 page:

 = t('.message')

403 page:

 = t('message_prefix') + t('.message').downcase_first	

While there are other, arguably better, ways of implementing something like this, #downcase_first would have been convenient.

The comment examples for #upcase_first indicate that the method was introduced for convenience in adjusting verbiage, and in our case, we wanted the same convenience in the opposite direction.

[natematykiewicz]

For what it's worth, it's generally not a good idea to concatenate strings for I18n. In lot of languages = t('message_prefix') + t('.message').downcase_first doesn't actually make grammatical sense. With I18n, it's best to have a single key for the entire message.

[rafaelfranca]

I didn't see any mention of #upcase_first in the Inflections section of the Active Support Core Extentions page. I'm happy to add documentation of both #upcase_first and #downcase_first if desired.

Let's add that.

[markaschneider]

I didn't see any mention of #upcase_first in the Inflections section of the Active Support Core Extentions page. I'm happy to add documentation of both #upcase_first and #downcase_first if desired.

Let's add that.

Added.