Fix many React warnings

src/components/fields/SpecField.jsx

@@ -76,6 +76,7 @@ export default class SpecField extends React.Component {
         } else {
           return <SelectInput
             {...commonProps}
+            value={this.props.value || this.props.fieldSpec.default}
             options={options}
           />
         }

src/components/fields/ZoomSpecField.jsx

@@ -182,7 +182,7 @@ export default class ZoomSpecField extends React.Component {
           <DocLabel
             label={<FunctionIcon />}
             cursorTargetStyle={{ cursor: 'pointer' }}
-            doc="Turn property into a zoom function to enable a map feature to change with map's zoom level."
+            doc={"Turn property into a zoom function to enable a map feature to change with map's zoom level."}
           />
         </Button>
         }