BUG #15048: pg_trgm extension: <% and %> operators ignore word boundaries

The following bug has been logged on the website:

Bug reference: 15048
Logged by: Dmitry Ryabov
Email address: dmitry-ryabov@mail.ru
PostgreSQL version: 10.1
Operating system: Debian 9
Description:

Affected versions: 9.6, 10. How to reproduce:
set pg_trgm.word_similarity_threshold = 0.5;
set pg_trgm.similarity_threshold = 0.5;

select
/* space was ignored */
'startstop' <% 'xxxxstar stopyyyy', 'xxxxstar stopyyyy' %> 'startstop',
/* words tests */
'startstop' %> 'stopyyyy', 'startstop' %> 'xxxxstar',
'startstop' <% 'stopyyyy', 'startstop' <% 'xxxxstar',
'startstop' % 'stopyyyy', 'startstop' % 'xxxxstar';