assert.Equal and similar output should be explicit about type inequality

[bombela]

In assert.Equal(t, a, b) if a and b do not have the same type, the test will fail, but the output will be the same as a value equality test failure.

It is confusing, because on complex equality assertion, ones might not realize the problem is different type, not unequal values.

Consider this example:

type struct Foo { V int }
type Foos []Foo

assert.Equal(t, Foos{{1}, {2}}, Foos{{1}, {2})

compared to:

assert.Equal(t, Foos{{1}, {2}}, []Foo{{1}, {2})

The second case will fail with a "not equal" message.

Instead, assert.Equal and similar should output an explicit message about type inequality.

cc @abhinav @prashantv @breerly

[ernesto-jimenez]

There's a PR open about improving error messages: #327

[HaraldNordgren]

Could likely be closed since https://github.com/stretchr/testify/pull/327 was merged.