Strip 0x prefixed private key

[sc0Vu]

Fix #27 #28

Maybe #30?

It seems 0x prefixed private key break the ec sign.

[codecov]

Codecov Report

Merging #31 (534fa0d) into master (5b14653) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #31      +/-   ##
============================================
+ Coverage     88.80%   89.05%   +0.24%     
- Complexity       64       65       +1     
============================================
  Files             1        1              
  Lines           134      137       +3     
============================================
+ Hits            119      122       +3     
  Misses           15       15              

Impacted Files	Coverage Δ	Complexity Δ
src/Transaction.php	89.05% <100.00%> (+0.24%)	65.00 <0.00> (+1.00)

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