Add +capitalize+ option to Inflector.humanize

So strings can be humanized without being capitalized:

    'employee_salary'.humanize                    # => "Employee salary"
    'employee_salary'.humanize(capitalize: false) # => "employee salary"
Merge pull request #12789 from claudiob/humanize-without-capitalizing

Add +capitalize+ option to Inflector.humanize
instance_variables returns symbols, so we should use symbols in our list
these variables are also private
use slice to avoid range allocation
each_with_object on the view_assigns hash
use a set and reject to avoid array allocations
calculate the ivars to remove in advance as a set and cache them in a
constant.

`view_assigns` can use the precalculated sets and remove instance
variables without allocating any extra arrays
drop string allocations in the log subscriber
drop one more string allocation