fix adjacent quotation marks marshaling in multiline string

[AllenX2018]

From the spec:

str5 = """Here are three quotation marks: ""\"."""
str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""

The adjacent quotation marks which make the multiline string closed early should be escaped when marshaling.

[codecov]

Codecov Report

Merging #365 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
+ Coverage   94.98%   95.00%   +0.02%     
==========================================
  Files          10       10              
  Lines        2112     2121       +9     
==========================================
+ Hits         2006     2015       +9     
  Misses         65       65              
  Partials       41       41              

Impacted Files	Coverage Δ
tomltree_write.go	95.51% <100.00%> (+0.13%)	⬆️

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[pelletier]

Good catch, thanks for the patch!