`\t` error msgs bug and some codestyle tweaks

Minor bugs: `\t` were used inside single quoted strings.
Tweak: `a, b = c` is the same as `a = c[0]; b = [1] if c.length > 1`.
Minor tweak: `qwe if rty` form instead of multiline when `qwe` is only one line long.
Minor tweak: thanks to `#{}` interpolation we can omit `.to_s`.
Update sqlmap.rb
uses mult-assign & include? more readable
adds most rubocop cleanups, not all
Land 3607, sqlmap plugin cleanup

after some additional cleanup here:
SHA: a4f2fb218c86b976812c9d6e88397b6ead6a239c