Query: Include and ThenInclude should support collection navigation properties that are just IEnumerable<T>

[divega]

We learned yesterday while discussing #1463 with @anpete and @ajcvickers that the EF7 runtime does not constrain collection navigation properties to be of types that can be assigned to ICollection<T> but that IEnumerable<T> is generally sufficient.

The eager loading API should relax to support the same navigation property types.

[divega]

HasMany() already supports IEnumerable<TRelatedEntity> so this seems to be the only issue blocking having navigation properties that are statically of that type.

@ajcvickers @rowanmiller @anpete note that this is currently pri1 for RTM but I wonder if it is just a matter of adding some tests and changing the signature of this overload of ThenInclude() from:

public static IIncludableQueryable<TEntity, TProperty> 
    ThenInclude<TEntity, TPreviousProperty, TProperty>( 
        [NotNull] this IIncludableQueryable<TEntity, ICollection<TPreviousProperty>> source, 
        [NotNull] Expression<Func<TPreviousProperty, TProperty>> navigationPropertyPath) 

To

public static IIncludableQueryable<TEntity, TProperty> 
    ThenInclude<TEntity, TPreviousProperty, TProperty>( 
        [NotNull] this IIncludableQueryable<TEntity, IEnumerable<TPreviousProperty>> source, 
        [NotNull] Expression<Func<TPreviousProperty, TProperty>> navigationPropertyPath) 

[cbaxter]

@divega Based on the current state of dev branch, making the proposed change seems to work as you expected with no obvious side-effects.

I modified the overload of ThenInclude in a local build and verified the existing tests pass via the official build script (although Azure tests skipped in my environment). I also ran through several ThenInclude examples in my test project and everything seems to be working like a charm.

Although a small change, this would be a nice addition to help keep entities from having to expose a public ICollection<T> member for use with Include references.

[ajcvickers]

Talked to @divega @rowanmiller and decided to take this post RTM, probably in first point release. We should also make navigations exposed as IEnumerables but backed by ICollections work at that time--they currently don't work.