Fix false positives about double quotes

[pocke]

StringLiterals, UnneededCapitalW and UnneededPercentQ cops have
false positives about double quotes.
If escaped normal character that is like \! exists in a literal, those
cops make false positives.

For examples

UnneededPercentQ

%Q{"foo\!"}

The above code can't be replaced with '"foo\!"'.

%Q{"foo\!"} == '"foo\!"'  # => false

But the cop adds an offense for the code.

$ rubocop --only UnneededPercentQ
Inspecting 1 file
C

Offenses:

test.rb:1:1: C: Use %Q only for strings that contain both single quotes and double quotes, or for dynamic strings that contain double quotes.
%Q{"foo\!"}
^^^^^^^^^^^

1 file inspected, 1 offense detected

UnneededCapitalW

%W(a \! b)

The above code can't be replaced with %w(a \! b)(small w).

%W(a \! b) == %w(a \! b)  # => false

But the cop adds an offense for the code.

$ rubocop --only UnneededCapitalW
Inspecting 1 file
C

Offenses:

test.rb:1:1: C: Do not use %W unless interpolation is needed. If not, use %w.
%W(a \! b)
^^^^^^^^^^

1 file inspected, 1 offense detected

StringLiterals

 # .rubocop.yml
StringLiterals:
  ConsistentQuotesInMultiline: true

"foo \!" \
  "\!"

The double quotes in the above code can't be replaced with single
quotes.
But the cop adds an offense for the code.

$ rubocop --only StringLiterals
Inspecting 1 file
C

Offenses:

test.rb:1:1: C: Prefer single-quoted strings when you don't need string interpolation or special symbols.
"foo \!" \ ...
^^^^^^^^^^

1 file inspected, 1 offense detected

---

The cause about this false positives is double_quotes_acceptable? method.
The method receives a string value. For example, when a literal is "\!", the method receives "!". So, the presence or absence of escape will be lost.

This change fixed the problems.

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).