Fix zero-length range options

[vweevers]

See #155.

• Prerequisite: Range tests on zero-length buffer+string are incompatible with bytewise, charwise & IDB abstract-leveldown#318.
• Remove this:

level-js/iterator.js

Lines 54 to 58 in 0b1507f

	// Temporary workaround for Level/abstract-leveldown#318
	if ((Buffer.isBuffer(lower) || typeof lower === 'string') && lower.length === 0) lower = undefined
	if ((Buffer.isBuffer(upper) || typeof upper === 'string') && upper.length === 0) upper = undefined
	if ((Buffer.isBuffer(lowerOpen) || typeof lowerOpen === 'string') && lowerOpen.length === 0) lowerOpen = undefined
	if ((Buffer.isBuffer(upperOpen) || typeof upperOpen === 'string') && upperOpen.length === 0) upperOpen = undefined

• Add custom tests, for zero-length strings and buffers, to start