Returns a centered copy of +self+.

If integer argument +size+ is greater than the size (in characters) of +self+,
returns a new string of length +size+ that is a copy of +self+,
centered and padded on one or both ends with +pad_string+:

  'hello'.center(6)             # => "hello "               # Padded on one end.
  'hello'.center(10)            # => "  hello   "           # Padded on both ends.
  'hello'.center(20, '-|')      # => "-|-|-|-hello-|-|-|-|" # Some padding repeated.
  'hello'.center(10, 'abcdefg') # => "abhelloabc"           # Some padding not used.
  '  hello  '.center(13)        # => "    hello    "
  'こんにちは'.center(10)         # => "  こんにちは   "      # Multi-byte characters.

If +size+ is less than or equal to the size of +self+, returns an unpadded copy of +self+:

  'hello'.center(5)   # => "hello"
  'hello'.center(-10) # => "hello"

Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].
