Prefer _exec over _eval alternatives.

Eval'ing Ruby code is in general a risky practice. Starting in Ruby
1.8.7+, an alternative form of these methods has been provided which
only accepts a Ruby block. These tend to be safer as they do not
directly allow string interpolation as the `_eval` equivalents do.
Remove unused steps.
Replace superfluous `eval` with simple method.

The only purpose the `eval` served was to "undo" the previous `inspect`
of the value. This is a technique which is listed here:

    https://www.ruby-forum.com/topic/103775

Separate generating the raw value for an attribute and it's display
value. Keep the original `value_for` name for the display message.
Merge pull request #1051 from rspec/prefer-exec-over-eval

Prefer _exec over _eval alternatives.