Fix eager loading due to ServeStream rename

ServeFile was renamed to ServeStream in ruby-grape#1520. Calling Grape.eager_load!
would fail with:

```
uninitialized constant Grape::ServeFile (NameError)
```

.travis.yml

@@ -25,6 +25,10 @@ matrix:
       gemfile: gemfiles/rails_5.gemfile
     - rvm: 2.7.1
       gemfile: gemfiles/rails_6.gemfile
+    - rvm: 2.7.1
+      gemfile: Gemfile
+      script:
+        - bundle exec rspec spec/integration/eager_load
     - rvm: 2.7.1
       gemfile: gemfiles/multi_json.gemfile
       script:

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Features
 
+* [#2072](https://github.com/ruby-grape/grape/pull/2072): Fix eager loading due to ServeStream rename - [@stanhu](https://github.com/stanhu).
 * [#1520](https://github.com/ruby-grape/grape/pull/1520): Un-deprecate stream-like objects - [@urkle](https://github.com/urkle).
 * [#2060](https://github.com/ruby-grape/grape/pull/2060): Drop support for Ruby 2.4 - [@dblock](https://github.com/dblock).
 * [#2060](https://github.com/ruby-grape/grape/pull/2060): Upgraded Rubocop to 0.84.0 - [@dblock](https://github.com/dblock).

lib/grape/eager_load.rb

@@ -16,5 +16,5 @@
 Grape::DSL.eager_load!
 Grape::API.eager_load!
 Grape::Presenters.eager_load!
-Grape::ServeFile.eager_load!
+Grape::ServeStream.eager_load!
 Rack::Head # AutoLoads the Rack::Head

spec/integration/eager_load/eager_load_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+$LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', '..', 'lib'))
+require 'grape'
+require 'grape/eager_load'
+
+describe 'eager loading' do
+  class API < Grape::API
+  end
+
+  it 'loads successfully' do
+    expect { Grape.eager_load! }.to_not raise_error
+    expect { API.compile! }.to_not raise_error
+  end
+end