Fix 14

[sc0Vu]

Change

1. Return Buffer([]) when input is 0, negative number, null.
2. Add tests for issue 14 Integer 0 should be encoded to ox80? #14 .
3. Remove mediumint and bigint test (loss of precision, the problem also in rlp js library).

[codecov-io]

Codecov Report

Merging #15 into master will decrease coverage by 0.33%.
The diff coverage is 81.81%.

@@             Coverage Diff              @@
##             master      #15      +/-   ##
============================================
- Coverage     81.12%   80.78%   -0.34%     
- Complexity       76       80       +4     
============================================
  Files             2        2              
  Lines           196      203       +7     
============================================
+ Hits            159      164       +5     
- Misses           37       39       +2

Impacted Files	Coverage Δ	Complexity Δ
src/RLP.php	63.46% <81.81%> (+0.57%)	36 <0> (+4)	⬆️

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