Add Form::set_boundary to customize multipart boundary

Customizing the boundary makes it easier to write tests that assert on the Content-Type header and body, as the content can be static. Method documentation sufficiently describes the risk of doing this. Closes #2374

Author: LucasPickering

src/async_impl/multipart.rs

@@ -72,6 +72,17 @@ impl Form {
         self.inner.boundary()
     }
 
+    /// Set the boundary that this form will use. By default the boundary is a
+    /// long random string to minimize the risk of the boundary appearing in
+    /// the body content.
+    ///
+    /// **Setting a custom boundary incurs significant risk of generating
+    /// corrupted bodies.** Only use this if you need it and you understand the
+    /// risk!
+    pub fn set_boundary(&mut self, boundary: impl Into<String>) {
+        self.inner.boundary = boundary.into();
+    }
+
     /// Add a data field with supplied name and value.
     ///
     /// # Examples

src/blocking/multipart.rs

@@ -79,6 +79,17 @@ impl Form {
         self.inner.boundary()
     }
 
+    /// Set the boundary that this form will use. By default the boundary is a
+    /// long random string to minimize the risk of the boundary appearing in
+    /// the body content.
+    ///
+    /// **Setting a custom boundary incurs significant risk of generating
+    /// corrupted bodies.** Only use this if you need it and you understand the
+    /// risk!
+    pub fn set_boundary(&mut self, boundary: impl Into<String>) {
+        self.inner.boundary = boundary.into();
+    }
+
     /// Add a data field with supplied name and value.
     ///
     /// # Examples