whitelist floats with exponents in YAML

fixes CNVS-26342

test plan:
 * have a quiz submission with an absurdly small score (like 1e-15);
   I have no idea how to produce this
 * loading speedgrader for that quiz should work

Change-Id: Icc7a73ab61d2517661824b828fb2a73f115a097e
Reviewed-on: https://gerrit.instructure.com/70024
Tested-by: Jenkins
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

lib/canvas_yaml.rb

@@ -49,6 +49,7 @@ SafeYAML::OPTIONS.merge!(
     whitelisted_tags: %w[
         tag:ruby.yaml.org,2002:symbol
         tag:yaml.org,2002:float
+        tag:yaml.org,2002:float#exp
         tag:yaml.org,2002:str
         tag:yaml.org,2002:timestamp
         tag:yaml.org,2002:timestamp#iso8601

spec/lib/safe_yaml_spec.rb

@@ -45,6 +45,7 @@ hwia: !map:HashWithIndifferentAccess
   b: 2
 float: !float
   5.1
+float_with_exp: -1.7763568394002505e-15
 os: !ruby/object:OpenStruct
   modifiable: true
   table: 
@@ -105,6 +106,9 @@ YAML
     float = verify(result, 'float', Float)
     expect(float).to eq 5.1
 
+    float_with_exp = verify(result, 'float_with_exp', Float)
+    expect(float_with_exp).to eq(-1.7763568394002505e-15)
+
     os = verify(result, 'os', OpenStruct)
     expect(os.a).to eq 1
     expect(os.b).to eq 2