[Codeplex Issue #1782]: Suppress key convention once [Key] explicitly
apply at *any* properties.

In OData Convention Model Builder, it will the property
that is annotated with the Key attribute and follows the "schema Id |
TypeName + Id".

We should suppress the convention rule if any properties were annotated
with [key].

Here is the example which can illustrate this modification:

Suppose we have the CLR type as:
=========================================
public class Customer
{
[Key]
public string Path { get; set; }
public long CustomerId { get; set; }
...
}

Then, we use OData Convention Model builder to build the EdmModel:
=========================================
ODataConventionModelBuilder builder = new ODataConventionModelBuilder();
builder.EntitySet<Customer>("Customers");
....
builder.GetEdmModel();

In $metadata, there are two keys existed:
=========================================
<EntityType Name="Customer">
<Key>
<PropertyRef Name="CustomerId" />
<PropertyRef Name="Path" />
</Key>
<Property Name="Path" Type="Edm.String" Nullable="false" />
<Property Name="CustomerId" Type="Edm.Int64" Nullable="false" />

Originally, User is explicitly decorated [Path] as the key, so, the
[CustomerId] shouldn't treat as key. So, this modification will suppress key
convention once [Key] explicitly apply at *any* properties.