safeMarshal should escape the error message

[shogo82148]

Issue #, if available:

the safeMarshal function might return invalid JSON if err.Error() contains special characters of JSON. ref #326

aws-lambda-go/lambda/invoke_loop.go

Lines 103 to 109 in 55dc88b

	func safeMarshal(v interface{}) []byte {
	payload, err := json.Marshal(v)
	if err != nil {
	return []byte(fmt.Sprintf(serializationErrorFormat, err.Error()))
	}
	return payload
	}

Description of changes:

use json.Marshal instead of fmt.Sprintf.

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[codecov-commenter]

Codecov Report

Merging #327 into master will increase coverage by 0.22%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
+ Coverage   72.23%   72.46%   +0.22%     
==========================================
  Files          18       18              
  Lines         724      730       +6     
==========================================
+ Hits          523      529       +6     
  Misses        136      136              
  Partials       65       65              

Impacted Files	Coverage Δ
lambda/invoke_loop.go	75.00% <71.42%> (+3.00%)	⬆️

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