tx: use io.SeekStart in WriteTo function

[lorneli]

Const os.SEEK_SET is deprecated. See https://golang.org/pkg/os/#pkg-variables.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@4ff482b). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master      #43   +/-   ##
=========================================
  Coverage          ?   84.81%           
=========================================
  Files             ?        9           
  Lines             ?     1851           
  Branches          ?        0           
=========================================
  Hits              ?     1570           
  Misses            ?      167           
  Partials          ?      114

Impacted Files	Coverage Δ
tx.go	76% <0%> (ø)

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[xiang90]

lgtm.

defer to @heyitsanthony

[heyitsanthony]

lgtm