Fix sending sanitizer class name in amp_sanitize Server-Timing headers

[westonruter]

I found in #990 (comment) that the Server-Timing headers are incorrect for the sanitizers, with erroneously repeating AMP_Tag_And_Attribute_Sanitizer for each sanitizer:

Server-Timing: amp_output_buffer;desc="AMP Output Buffer";dur=325.990915
Server-Timing: amp_dom_parse;desc="AMP DOM Parse";dur=4.322052
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.546932
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=6.416798
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.148058
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.276089
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.092030
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.113010
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.106812
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.201941
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.336885
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.022888
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.175953
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.034809
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=0.024796
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=38.730860
Server-Timing: amp_sanitize;desc="AMP_Tag_And_Attribute_Sanitizer";dur=81.669092
Server-Timing: amp_dom_serialize;desc="AMP DOM Serialize";dur=7.952929

This is due to a bug I introduced in 7a218aa for #1175.