Fix #12

[sc0Vu]

Change
Add attribute map
Check attribute when set the value

For more detail, see #12

[codecov-io]

Codecov Report

Merging #13 into master will decrease coverage by 0.79%.
The diff coverage is 75%.

@@             Coverage Diff             @@
##             master      #13     +/-   ##
===========================================
- Coverage     86.61%   85.82%   -0.8%     
- Complexity       56       67     +11     
===========================================
  Files             1        1             
  Lines           127      134      +7     
===========================================
+ Hits            110      115      +5     
- Misses           17       19      +2

Impacted Files	Coverage Δ	Complexity Δ
src/Transaction.php	85.82% <75%> (-0.8%)	67 <0> (+11)

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