Fix indent for merge_imports

Example
---
```rust
mod tests {
    use foo$0::bar;
    use foo::baz;
    fn feature() {}
}
```

**Before this PR**

```rust
mod tests {
    use foo::{bar, baz};
        fn feature() {}
}
```

**After this PR**

```rust
mod tests {
    use foo::{bar, baz};
    fn feature() {}
}
```

Author: A4-Tacks

crates/ide-assists/src/handlers/merge_imports.rs

@@ -563,6 +563,26 @@ use foo::{bar, baz};
         );
     }
 
+    #[test]
+    fn mod_indent_whitespace() {
+        check_assist(
+            merge_imports,
+            r"
+mod tests {
+    use foo$0::bar;
+    use foo::baz;
+    fn feature() {}
+}
+",
+            r"
+mod tests {
+    use foo::{bar, baz};
+    fn feature() {}
+}
+",
+        );
+    }
+
     #[test]
     fn works_with_trailing_comma() {
         check_assist(

crates/syntax/src/syntax_editor/edits.rs

@@ -221,7 +221,7 @@ impl Removable for ast::Use {
             .and_then(ast::Whitespace::cast);
         if let Some(next_ws) = next_ws {
             let ws_text = next_ws.syntax().text();
-            if let Some(rest) = ws_text.strip_prefix('\n') {
+            if let Some(rest) = ws_text.trim_end_matches(' ').strip_prefix('\n') {
                 if rest.is_empty() {
                     editor.delete(next_ws.syntax());
                 } else {