XML encoded FHIR resource created with decimal element that has leading plus sign value couldn't be retrieved or modified

[volodymyr-korzh]

It is possible to create resource with decimal element that has leading plus sign using XML encoding.
However, this resource couldn't be modified/retrieved/deleted to fix the leading plus sign.

1. Start an instance of HAPI-FHIR server with disabled request validation for the FHIR endpoint.
2. POST an xml bundle with a resource that has a leading plus sign in a decimal element:

Bundle with Observation

POST: http://localhost:8000/
Content-Type : application/xml

<Bundle
	xmlns="http://hl7.org/fhir" >
	<type value="transaction"/>
	<entry>
		<fullUrl value="urn:uuid:d403a26d-d5d3-47a0-8cac-0511757c5f51"/>
		<resource>
			<Observation>
				<identifier>
					<system value="http://test.com/fhir/test"/>
					<value value="o_test"/>
				</identifier>
				<status value="preliminary"/>
				<category>
					<coding>
						<system value="http://terminology.hl7.org/CodeSystem/observation-category"/>
						<code value="vital-signs"/>
						<display value="Vital Signs"/>
					</coding>
				</category>
				<code>
					<coding>
						<system value="http://loinc.org"/>
						<code value="29463-7"/>
						<display value="Body Weight"/>
					</coding>
				</code>
				<referenceRange>
					<low>
						<value value="-3.0"/>
						<unit value="mmol/l"/>
						<system value="http://Home" />
						<code value="mmol/l"/>
					</low>
					<high>
						<value value="+3.0"/>
						<unit value="mmol/l"/>
						<system value="http://Home" />
						<code value="mmol/l"/>
					</high>
					<text value="-3.0 mmol/l - +3.0 mmol/l"/>
				</referenceRange>
			</Observation>
		</resource>
		<request>
			<method value="PUT"/>
			<url value="Observation?identifier=http://test.com/fhir/test|o_test"/>
		</request>
	</entry>
</Bundle>

3. Execute GET operation for created Observation:
   GET: http://localhost:8000/Observation/3002

Expected behaviour:
Observation successfully retrieved and decimal element referenceRange.high.value has correct value (3.0 instead of +3.0):

	<referenceRange>
		...
		<high>
			<value value="3.0"/>
			<unit value="mmol/l"/>
			<system value="Home"/>
			<code value="mmol/l"/>
		</high>
		<text value="-3.0 mmol/l - +3.0 mmol/l"/>
	</referenceRange>

Actual result:
Observation couldn't be retrieved, error is returned.

<OperationOutcome
	xmlns="http://hl7.org/fhir">
	<issue>
		<severity value="error"/>
		<code value="processing"/>
		<diagnostics value="HAPI-2223: HAPI-0928: Failed to parse database resource[Observation/3002 (pid 3002, version R4): HAPI-1861: Failed to parse JSON encoded FHIR content: Unexpected character ('+' (code 43)) in numeric value: JSON spec does not allow numbers to have plus signs: enable `JsonReadFeature.ALLOW_LEADING_PLUS_SIGN_FOR_NUMBERS` to allow&#xa; at [line: 1, column: 472]"/>
	</issue>
</OperationOutcome>