[BREKING] Make Close() interface the same as that of httptest

[k1LoW]

No description provided.

[github-actions]

Code Metrics Report

	main (5ba411d)	#4 (738d48e)	+/-
Coverage	84.3%	85.3%	+1.0%
Code to Test Ratio	1:0.5	1:0.5	+0.0
Test Execution Time	22s	23s	+1s

Details

  |                     | main (5ba411d) | #4 (738d48e) |  +/-  |
  |---------------------|----------------|--------------|-------|
+ | Coverage            |          84.3% |        85.3% | +1.0% |
  |   Files             |              1 |            1 |     0 |
  |   Lines             |             70 |           68 |    -2 |
- |   Covered           |             59 |           58 |    -1 |
+ | Code to Test Ratio  |          1:0.5 |        1:0.5 |  +0.0 |
  |   Code              |            162 |          159 |    -3 |
  |   Test              |             81 |           81 |     0 |
- | Test Execution Time |            22s |          23s |   +1s |

Code coverage of files in pull request scope (84.3% → 85.3%)

Files	Coverage	+/-
smtptest.go	85.3%	+1.0%

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Reported by octocov