[buffer] fill() for utf16le is broken

[trevnorris]

• Version: v6.9.1
• Platform: Linux
• Subsystem: buffer

Example of brokenness:

Buffer.allocUnsafeSlow(16).fill('a', 'utf16le').toString('utf16le')
// expect: 'aaaaaaaa'
// get:    '慡慡慡慡慡慡慡慡'

Seems to be treating the character as latin1.

[trevnorris]

Thanks @addaleax!