WebSQL : Fix getSchema when having more than one multi-entry index.

[mehdirande]

In WebSQL implementation multi-entry indexes are stored in a separate table. Each 'multientry' table is composed of two columns, one which name match the multi-entry index name and one column 'id'. What we expect getSchema method to do is : When parsing a 'multientry' table,

• If there already is an entry in the stores map, add the multi-entry index to the entry
• if not, create an entry in the stores map with only the multi-entry index When parsing the 'store' table :
• If there already is an entry in the stores map, merge all indexes of the entry with those of the 'store' table. What getSchema was doing : When parsing a 'multientry' table :
• If there already is an entry in the stores map, replace all indexes of the entry by the multientry table indexes (id + multientry index)
• if not, it works as expected. When parsing the 'store' table :
• if there already is an entry in the stores map, add only the first recorded index to the store indexes So if :
• Store is parsed before the multientry table (which happens rarely ) the store indexes are overwritten by the multientry table indexes.
• There is more than one multi-entry index, only one of the multi entry indexes is added to the store description.

[yathit]

@mehdirande Thanks for great PR. I have changed a bit so that it fit with strangest compile flag and some failing cases on other tests. Please add unit test if you are in doubt.