Handle more unsafe String methods

[jaynetics]

Summary

• adds recently introduced String mutation methods #delete_prefix(!), #delete_suffix(!), and #unicode_normalize(!) to UNSAFE_STRING_METHODS:

# before
string = '<!--evil-->'.html_safe
string.delete_prefix!('<!--')
string.html_safe? # => true

# after
string = '<!--evil-->'.html_safe
string.delete_prefix!('<!--')
string.html_safe? # => false

• treats String combination methods #[]=, #insert and #replace the same way #prepend and #concat are already treated:

# before
string = 'foo'.html_safe
string[0] = '<b>'
string # => "<b>oo"
string.html_safe? # => true

# after
string = 'foo'.html_safe
string[0] = '<b>'
string # => "&lt;b&gt;oo"
string.html_safe? # => true

• greatly improves test coverage of unsafe String methods

• replaces the obscure unsafe_method.respond_to?(unsafe_method) with the faster String.method_defined?(unsafe_method).

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[rafaelfranca]

Speed is not a problem here. This loop only run once, UNSAFE_STRING_METHODS times, so I prefer to keep the respond_to?

[jaynetics]

@rafaelfranca OK, changed back.

[rafaelfranca]

Thanks!