**Note**: \Method +pread+ is different from other reading methods
in that it does not modify +self+ in any way;
thus, multiple threads may read safely from the same stream.

Reads up to +maxlen+ bytes from the stream,
beginning at 0-based byte offset +offset+;
returns a string containing the read bytes.

The returned string:

- Contains +maxlen+ bytes from the stream, if available;
  otherwise contains all available bytes.
- Has encoding +Encoding::ASCII_8BIT+.

With only arguments +maxlen+ and +offset+ given,
returns a new string:

  english = 'Hello'  # Five 1-byte characters.
  strio = StringIO.new(english)
  strio.pread(3, 0)  # => "Hel"
  strio.pread(3, 2)  # => "llo"
  strio.pread(0, 0)  # => ""
  strio.pread(50, 0) # => "Hello"
  strio.pread(50, 2) # => "llo"
  strio.pread(50, 4) # => "o"
  strio.pread(0, 0).encoding
  # => #<Encoding:BINARY (ASCII-8BIT)>

  russian = 'Привет' # Six 2-byte characters.
  strio = StringIO.new(russian)
  strio.pread(50, 0) # All 12 bytes.
  # => "\xD0\x9F\xD1\x80\xD0\xB8\xD0\xB2\xD0\xB5\xD1\x82"
  strio.pread(3, 0)  # => "\xD0\x9F\xD1"
  strio.pread(3, 3)  # => "\x80\xD0\xB8"
  strio.pread(0, 0).encoding
  # => #<Encoding:BINARY (ASCII-8BIT)>

  japanese = 'こんにちは' # Five 3-byte characters.
  strio = StringIO.new(japanese)
  strio.pread(50, 0)    # All 15 bytes.
  # => "\xE3\x81\x93\xE3\x82\x93\xE3\x81\xAB\xE3\x81\xA1\xE3\x81\xAF"
  strio.pread(6, 0)     # => "\xE3\x81\x93\xE3\x82\x93"
  strio.pread(1, 2)     # => "\x93"
  strio.pread(0, 0).encoding
  # => #<Encoding:BINARY (ASCII-8BIT)>

Raises an exception if +offset+ is out-of-range:

  strio = StringIO.new(english)
  strio.pread(5, 50) # Raises EOFError: end of file reached

With string argument +out_string+ given:

- Reads as above.
- Overwrites the content of +out_string+ with the read bytes.

Examples:

  out_string = 'Will be overwritten'
  out_string.encoding                # => #<Encoding:UTF-8>
  result = StringIO.new(english).pread(50, 0, out_string)
  result.__id__ == out_string.__id__ # => true
  out_string                         # => "Hello"
  out_string.encoding                # => #<Encoding:BINARY (ASCII-8BIT)>

