lexically_relative should ignore trailing slash on base

**Expected behavior**
```c++
fs::path("a/b").lexically_relative("a") == "b";
fs::path("a/b").lexically_relative("a/") == "b";
```

**Actual behavior**
```c++
fs::path("a/b").lexically_relative("a/") == "b";
fs::path("a/b").lexically_relative("a/") == "../b";
```
(tested on AppleClang 11.0.0 on macOS 10.15.3)