[5.3] Fix morphTo naming inconsistency

[themsaid]

As reported here: #10501

Currently the relationship name is converted to snake_case, which is inconsistent with other relationship types naming.

[joshbodine21]

This modification seems to have caused an issue with the possibility of custom naming of morphTo relationships. For example, before this change, in my code I was able to use $table->morphs('Affiliation') and it would create my DB columns as expected (Affiliation_id and Affiliation_type). This change now causes the code to look for affiliation_type and _id which doesn't match my case. I realize that may not be to convention, but previously I was able to pass my parameters to the morphTo relationship and my naming was honored. Now, it forces the snake_case and my morphTo relationship returns null.