Added:

• removeKey(String key); returns void
  
• getKeys(); returns Set<String>

Tested in versions:

• 1.7.10 (Why are you still using 1.7?!?)
• 1.8.8
• 1.9
• 1.9.2
• 1.9.4
  
• 1.10
• 1.10.2

Notes:
Using setString("key", null); acts like remove("key");