Use common snippet name prefix as operationId, fix #140

[ozscheyge]

See #140 for problem description.

Solution

Example 1

Snippet IDs have a common prefix:

get-product
get-product_xml
get-product_with_extra_attributes

The resulting operationId would be the common prefix get-product

Example 2

Snippet IDs do not have a common prefix:

first
second
last

The resulting operationId would be the snippet IDs sorted lexicographically and then concatenated resulting in firstlastsecond

[coveralls]

Coverage increased (+0.03%) to 92.63% when pulling dd56608 on new/use_common_prefix_as_operationId into 022cfe4 on master.